French President Nicolas Sarkozy and his wife Carla Bruni-Sarkozy are in Britain for the start of a rare state visit, stressing that the West must beat the Taliban in Afghanistan, no matter what the cost.

Addressing Britain's House of Commons and House of Lords, Sarkozy follows a powerful tribute to Britain's role in countering the threat of fascism in Europe, and promise to stand by London's side as it fights the Taliban in Afghanistan.

"We cannot afford to lose Afghanistan. We cannot afford to see the Taliban and Al-Qaida returning to Kabul, defeat is not an option, however difficult the victory may be."

He says he will pledge more French troops for NATO's mission in Afghanistan during a summit next week in Bucharest, Romania - if allies offer Afghans more responsibility and better coordinate non-military efforts.

The offer underscores his determination to revitalise France's relations with Britain and the United States.

Sarkozy's 36-hour trip is the first state visit to Britain by a French president in 12 years.